Electrocoating bath compositions

ABSTRACT

An electrocoating bath composition comprises a diluted aqueous solution or dispersion of a heat curable coating resin, suitably an epoxy ester resin, and a minor amount of water soluble, compatible resin which cures rapidly at temperatures substantially below the curing temperature of the coating resin, for example a water soluble phenolic resin, to improve the edge and corner coverage of objects coated by the composition.

' [22] Filed:

United States Patent 1 Crowne et a1.

[54] ELECTROCOATING BATH COMPOSITIONS [75] Inventors: Francis Raymond Crowne, Oakville, Ontario, Canada; Vladimir Peer,

Bloomfield, NJ.

[73] Assignee: Canada Wire and Cable Company Limited, Toronto, Ontario, Canada Nov. 6, 1970 211 Appl. No.: 87,648

[52] US. Cl.. ..260/19 EP, 260/21, 260/292 EP [51] Int. Cl. ..C08g 5/21 [58] Field of Search ..260/19 EP, 19 N, 260/18CL,21,29.2

[5 6] References Cited UNITED STATES PATENTS 3,245,925 4/1966 Watson ..260/19 EP 1 May 29, 1973 2,810,674 10/1957 Madden ..260/19 EP 3,030,332 4/1962 Lombardi. ..260/19 EP 2,844,553 7/1958 Taylor ..260/19 EP Primary ExaminerM. J. Welsh Assistant Examiner-William B. Parker Attorney-Maybee and Legris [57] ABSTRACT 4 Claims, No Drawings ELECTROCOATING BATH COMPOSITIONS BACKGROUND OF THE INVENTION This invention relates to an electrocoating process, and to a bath composition for use in an electrocoating process.

Electrocoating is a process in which a coating film is applied to all surfaces of an object by the passage of an electric current between the object and another electrode, while immersed in a liquid bath containing the coating composition. Normally the liquid bath is an aqueous solution or suspension of the coating film ingredients, although in some cases non-aqueous liquid suspensions can be employed. Electrocoating is now a common way of applying paint to metal objects, for example to automobile bodies. Commonly, the object to be coated is made the anode and the bath structure itself is the cathode, direct current being passed therebetween.

In electrocoating processes, the objects to be coated are immersed in the dispersion of coating composition, coated by passage of the electric current, and removed from the bath. Excess coating which is clinging to the object merely as a result of the dipping operation is removed, and then the coated objects are heated to affix the coating permanently to the object. The coating usually comprises-a heat curable coating resin, which is cured into a permanent film by baking after its application to the object. An electrocoating process can be readily adapted for continuous operation, for example by transporting the objects to be coated continuously through the bath and into an oven.

BRIEF DESCRIPTION OF THE PRIOR ART Whilst the electrocoating process provides many desirable features as compared with other coating and painting processes, particularly in terms of ease of developing continuous operations, and efficient utilisation of coating compositions, there are associated problems. One of these problems, which the present invention sets out to solve, is that the final cured coating of the article tends to be of non-uniform thickness. In particular, the sharp edges and corners of the object tend to have a much thinner final coating than the rest of the surface of the object.

The normal way of increasing the thickness of the coating on the object in electrocoating is to increase the amount of current passed between the electrodes. However, it has been found that this does not result in a thicker final coating of resin on the sharp edges and corners of the object, but merely increases the resin coating thickness on the other parts of the surface. The provision of a final coating of adequate thickness on all surfaces of an. objectis particularly important where the coating is intended to provide electrical insulation of the object.

It would ordinarily be expected that the coating thickness at the sharp edges and corners of an object coated by an electrocoating process would in fact be thicker than the coating on the smooth surfaces. The coating composition will deposit preferentially at portions of the surface of the object electrodes having the highest electrical charge density, i.e., the sharp edges and corners.

SUMMARY OF THE INVENTION We have found that the root of the problem of adequate edge and corner coverage lies not in the actual electrocoating process, but in the subsequent behaviour of the coating prior to its final curing. Adequate coating onto the sharp edges and corners is apparently achieved in the bath, but the film tends to migrate away from these edges and corners between the time when the coated object is removed from the bath and the time when it is finally cured. This migration may be associated with the removal of water from the coating film, or perhaps with a certain stage of heating of the coating film, or even with a certain stage of cure of the resin. We have found that the problem may be overcome, or at any rate considerably reduced in magnitude, by providing a resin system which is curable without having the resin pass through a highly fluid state. To accomplish this we use small amounts of a compatible rapidly low temperature curing resin in conjunction with the resin which is basically to form the coating. The resin which is added cures prior to the curing of the coating resin and rapidly enough to prevent migration of the resin film away from the corners and edges during the critical period after coating the substrate and piror to final curing of the coating resin. The added resin appears to cure rapidly during this period, and in a sense provides a rigid chemical framework preventing flow of the uncured thermosetting resin prior to its own curing. The added resin may cure at room temperature; or at temperatures lower than that at which the coating resin cures.

The low temperature curable resins which are used in accordance with the present invention are those which have good compatability with the coating resin being used. When the coating resin is an epoxy ester resin, the low temperature curing resin is suitably a water soluble phenol-formaldehyde resin. The low temperature curing resin is suitably used in amounts from about 0.l to about 10 parts per parts of coating resin, and preferably from 1 to about 5 parts per 100 parts of coating resin, all parts being expressed by weight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One particularly useful application of this process is in connection with the electrocoating of aluminum metal strip to provide an electrically insulating coating of epoxy ester resin thereon, so as to render the strip suitable for use in large transformer windings. It will therefore be more particularly described with reference to this application. Thus in one preferred process, an aluminum metal strip is passed continuously through a bath composition containing coating resin and low temperature curing resin, as described above. In the bath, the strip is coated with an epoxy ester resin coating suitable to provide insulation of the aluminum strip, and the strip is then fed continuously through an oven in which the coating is baked and cured. By means of this process, aluminum strips can be coated continuously with an electrically insulating coating which is of sufficiently high dielectric strength, all over the surface of the strip, to allow use of the strip in transformer windings.

Epoxy ester resins are selected for use in this application on the basis of their desirable electrical and chemical properties for providing a durable, flexible, inert, heat resistant, electrically insulating coating on aluminum metal. The selected epoxy ester resin should be a high carboxylic acid content resin, solubilised by partial neutralisation, so that the coating bath comprises at least in part a solution of resin in water. Epoxy ester resins may be prepared by any of the well-known methods, for example by esterifying an epoxy resin with one or more fatty acids, to obtain a resin with a low acid value. In this method, the resulting epoxy ester resin may then be reacted with other quantities of organic acids, thereby increasing the acid number of the resin and bringing it into a suitable condition for subsequent preparation of the aqueous coating bath.

Preferred resins include the epoxyester resin produced by esterifying a solid epoxy resin derived from epichlorohydrin and bis phenol-A with dehydrated caster oil fatty acids to produce an epoxy ester with an acid value (mg. potassium hydroxide per gram of resin) of from about 8 to 10, and subsequently heating the resin with succinic anhydride until the acid value reaches about 50. Another preferred resin is that produced by esterifying the same solid epoxy resin with linseed oil fatty acids, and subsequently heating the resin with phthalic anhydride. In both cases, the resin solutions are then conveniently diluted, with organic hydroxyl solvents such as ethylene glycol monobutyl ether, or butyl alcohol, to about 76 percent solids, for storage and further use.

A typical electrocoating bath compsoition using an epoxy ester resin for this application, will contain from about parts by weight to about parts by weight of resin in 100 parts by weight of water.

The low temperature curing phenolic resin also appears to exert other beneficial effects on the final coating, besides solving the edge coating problem. For example, it appears to confer increased chemical resistance on the final cured film. As previously mentioned, the amount of such phenolic resin used is suitably from about 0.1 to about 10 parts, and preferably from about 1 to about 5 parts by weight, per 100 parts by weight of epoxy ester resin.

A preferred phenolic resin for this purpose is that produced by reacting 1 mole of phenol with a 37 percent aqueous solution containing 1.3 moles of formaldehyde, in the presence of a percent aqueous solution containing 20 millimoles of sodium hydroxide. This mixture is refluxed for 45 minutes and then cooled, and stored in a refrigerator ready for further use. The solution contains 60 percent solids, the resin being in its water soluble Resol stage. Another preferred phenolic resin, believed to be of the same type, is that sold under thetrade mark BRLY 1215 by Union Carbide Corporation.

Prior to its addition to the coating bath 50 percent of the acidity of the epoxy ester resin is neutralised by addition, under conditions of high shear, of the required amounts of an amine, such as triethylamine, dimethylethanolamine, or a mixture thereof. The amine is added to the solution of resin containing about 76 percent solids, referred to previously. The partially neutralised resin solution so formed is then diluted with water, again under conditions of high shear, and mixed with the aqueous dispersion of low temperature curing phenolic resin prepared as previously described. Then the bath is diluted if necessary to the desired solids content with deionized water. The coating bath is now ready for use in an electrocoating apparatus.

- The bath composition and process of the invention are illustrated in the following examples.

EXAMPLE 1.

An epoxy ester resin with good electrical insulating properties was prepared and rendered water soluble, and an aqueous electrocoating bath composition was made up from this resin. The bath composition was used to electrocoat an aluminum metal strip with the epoxy ester resin.

The epoxy ester resin was prepared as follows. 1435 grams (1 OH equivalent) of the commercially available epoxy resin Epon 1001 (derived from epichlorohydrin and bisphenol-A, and marketed by the Shell Chemical Company Limited) was mixed with 1085 grams (0.4 equivalents) of dehydrated castor oil fatty acids and 0.22 grams (0.425 milli-equivalents) of anhydrous sodium carbonate at a temperature of C under an atmosphere of nitrogen. The mixture was then heated. When the temperature reached C, dropwise addition of xylene was commenced, to facilitate the removal of the water of esterification generated during the reaction. The mixture was heated to a temperature of 220C, and maintained at this temperature for two to 3 hours, until the resin formed had an acid value.8 to 10 milligrams KOH per gram of resin. The reaction mixture was then cooled to 140C.

The epoxy ester resin so formed was then heated with more acid, to increase the carboxylic content of the resin. For this purpose, 199 grams (0.2 moles) of succinic anhydride was added to the resin, at 140C, and this reaction temperature was maintained for about 2 hours, until the resin had an acid value of 50 milligrams KOH per gram of resin. Then the reaction mixture was cooled to 110C.

The resultant resin was then thinned to 80 percent solids content by addition of 674 grams of ethylene glycol monobutyl ether and further thinned to 76 percent solids by addition of grams of butanol. In this condition, the solubilised epoxy ester resin can be stored for subsequent use.

A water soluble phenol-formaldehyde resin was also added to the coating bath. The resin used was that sold under the trade name BRLY 1215, by Union Carbide Corporation. It is believed to be prepared by heating an aqueous solution of phenol and formaldehyde under reflux until a resin in its water soluble, Resol stage is formed.

The electrocoating bath composition was then made up from these ingredients. All mixing of the various solutions and dispersions was carried out under conditions of high shear agitation. A portion of the thinned, 76 percent epoxy ester resin solution prepared as described above, containing 131 grams of resin, was neutralised 50 percent by addition of 4 grams (45 milliequivalents per 100 grams of resin) of dimethylethanolamine. To this partially neutralised resin solution was then added 3.5 grams of the 60 percent solids aqueous solution of phenolic resin and sufficient additional deionised water to reduce the solids content of the bath composition to about 10 percent.

The bath composition thus contained 10 percent of epoxy ester and 0.2 percent of the water soluble phenolic resin. The composition had a milky appearance. It showed a conductivity of about 1000 umho, and a pH value of about 7.5.

This bath composition was used to electrocoat a piece of aluminum metal strip, in a small scale, batch type experiment. The piece of aluminum, which had a thickness of l mils and a surface area (one side) of 3 square inches, was immersed in the bath composition and connected in as the anode of an electrical circuit. The cathode of the circuit comprised a steel plate having a surface area of (one side) 4 square inches, immersed in the bath composition at a distance of about 0.5 inches from the aluminum metal anode. A direct electric current of 125 volts and about 0.1 amps was passed between the anode and the cathode for a period of 20 seconds, causing deposition of a translucent white coating on the aluminum anode. The piece of aluminum was then removed from the bath composition, washed with water, dried for 30 minutes at 150C and baked for 5 minutes at 250C. The resulting cured resin coating was yellow in colour, and had a thickness of 0.95 mils.

The dielectric strength of the applied coating was tested, by applying a potential difference between the aluminum (beneath the coating) and a mobile electrical probe. By moving the electrical probe over all parts of the surface of the aluminum piece, including the sharp edges and corners thereof, it was found that the coating had a dielectric strength of greater than 1100 volts at all the locations, indicating good surface, edge and corner coverage. The film had a smooth, glossy appearance.

Whilst in the above example the electrocoating bath composition was used in a small scale, batch type laboratory experiment to show its efficiency, it will be apparent that for large scale operation, the same composition could be used in a continuous system, in which the composition is contained in a metal bath containing cathodes for the operation, and the aluminum strip anode is of great length and is fed continuously therethrough. This is the preferred commercial process. After emerging from the bath, the strip is continuously fed under an air wipe and through an oven in which it is dried and cured.

This is a copolymer of butadiene and a substituted styrene homologue, namely a-methylstyrene to tetramer, of molecular weight about 3000, comprising about 93 wt. butadiene, and having carboxylic acid end groups so that it is water soluble.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrocoating bath composition consisting essentially of a dilute aqueous solution or dispersion of parts by weight of a heat curable, water soluble or dispersible, epoxy ester coating resin, and from about 0.1 to about 10 parts by weight of a phenol aldehyde resin in water soluble Resol form which is substantially compatible with said coating resin, and which is rapidly curing at temperatures below the .curing temperature of the coating resin.

2. The composition of claim 1 wherein the phenolic resin is used in an amount of from about 1 to about 5 parts by weight, per 100 parts by weight of heat curable coating resin.

3. The composition of claim 1 wherein the epoxy ester resin is that produced by esterifying a solid epoxy resin derived from epichlorohydrin and P,P- isopropylidenediphenol with dehydrated castor oil fatty acids, and solubilized by acidifying with an organic acid to increase the acid number thereof and partially neutralizing with an amine.

4. The composition of claim 2 wherein the phenolic resin is that produced by reacting phenol with excess formaldehyde under aqueous alkaline conditions, until the resin is in its water soluble Resol stage. 

2. The composition of claim 1 wherein the phenolic resin is used in an amount of from about 1 to about 5 parts by weight, per 100 parts by weight of heat curable coating resin.
 3. The composition of claim 1 wherein the epoxy ester resin is that produced by esterifying a solid epoxy resin derived from epichlorohydrin and P,P''-isopropylidenediphenol with dehydrated castor oil fatty acids, and solubilized by acidifying with an organic acid to increase the acid number thereof and partially neutralizing with an amine.
 4. The composition of claim 2 wherein the phenolic resin is that produced by reacting phenol with excess formaldehyde under aqueous alkaline conditions, until the resin is in its water soluble Resol stage. 